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The phenology of vegetation, which describes the seasonal evolution of plants, can be effectively monitored from space. This approach offers important advantages compared to field observations, as quantitative information can be derived for any location worldwide over a number of years, thereby offering a consistent overview of the fate of the observed biomes and their relations with the climate and the environment. This manuscript describes a method to define the start, end, and length of ‘growing seasons’ based on the statistical analysis of time series of the biogeophysical quantity known as the Fraction of Absorbed Photosynthetically Active Radiation derived from an analysis of SeaWiFS data. Results are discussed for various biomes.

This paper describes the evaluation and performance of the Medium Resolution Imaging Spectrometer (MERIS) Global Vegetation Index (MGVI) algorithm that is implemented in the MERIS ground segment as the primary land surface product. MGVI output values represent the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) which acts as an indicator of the presence and state of the vegetation canopy. The retrieval algorithm was optimized to be insensitive to the overlying atmosphere, the underlying soil as well as angular effects, using radiative transfer models. This physically based approach for retrieving land biophysical parameters can be extended to a series of sensors; the resulting algorithms are designed to deliver similar geophysical products that are directly comparable and ultimately generate long time series of FAPAR. After presenting the MGVI algorithm, we analyze actual results by inter-comparing the FAPAR values derived from MERIS to similar products derived from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) that have been generated at the European Commission Joint Research Centre (EC-JRC). The FAPAR products derived from MERIS and SeaWiFS are compared on several dates in 2002 when both instruments simultaneously observed the same geographical regions. The consistency and temporal continuity of the products are then evaluated by comparing the time series of FAPAR values over local sites in Europe during the year 2003 from the two instruments. Finally, multi-annual FAPAR time series obtained by merging MERIS and SeaWiFS products demonstrate the feasibility of monitoring the state of land surfaces with FAPAR products before and after the European drought event in 2003.

The state of terrestrial vegetation has been monitored using remote sensing data for decades. Information was often derived from empirical tools, like vegetation indices, which are very sensitive to perturbations and often depend on the spectral properties of the sensor. Advances in the understanding of radiation transfer and the availability of higher performance instruments have stimulated the development of a new generation of geophysical products poised to provide reliable, accurate information on the state and evolution of terrestrial environments. A series of optimized algorithms have been developed for documenting biophysical activities, using a physically based approach (specifically, to estimate Fraction of Absorbed Photosynthetically Active Radiation (FAPAR)) for various instruments. The outline of the methodology will be summarized and the results from an application conducted with SeaWiFS data will be presented.